The present invention relates to a process for the preparation of protein isolates having improved physical characteristics such as color, nutritional value and/or palatability (including flavor, taste and/or odor) from vegetable protein sources such as vegetable meals and flours. It particularly relates to an improved process for the preparation of protein isolates from vegetable protein sources of the type in which the vegetable protein source is contacted with alkali to extract protein therefrom, and the resulting aqueous protein extract phase is thereafter precipitated with acid to produce a protein isolate.
When oil is expelled from oilseeds, such as for example soybeans, sunflower seeds, etc., by techniques well known to those skilled in the art, a resulting by-product is a protein-containing material which is known as a vegetable meal, e.g. soybean meal, sunflower meal, etc. As is also well known in the art, this vegetable meal may be ground to produce a flour, typically containing about 50 percent protein. The vegetable meal and/or flour, due to its high protein content, has the potential of being a valuable nutritional source, capable of use as additives in a variety of food and feed applications. Sunflower protein is particularly desirable in this regard because of the well-balanced profile of essential amino acids (except for low lysine content), the absence of any known anti-nutritional factors, and relatively good flavor characteristics.
One method heretofore known for isolating protein from vegetable protein sources such as various oil seed meals and flours involves alkali extraction of the meal to extract protein therefrom followed by acid precipitation of protein from the aqueous protein extract phase. In the alkali extraction step, the meal is first admixed with water, the weight ratio of water to meal ordinarily ranging from about 10:1 to 30:1, typically comprising about 20:1. The pH of the water is then adjusted to range from 8.5 to 10 by the addition of a strong base, for example, an alkali metal hydroxide such as sodium hydroxide. The meal/alkaline water mixture is then agitated, typically for about one hour, whereby protein and some non-protein impurities are extracted from the meal to form a liquid protein extract phase and a solid residue of spent meal. The extract phase which is a solution comprising water, protein, and non-protein impurities is then separated from the spent meal, for example, by centrifugation. This extract phase is then treated in an acid precipitation step. In this step, the pH of the extract phase is adjusted to range from 4.5 to 4.7 by the addition of an acid, for example, hydrochloric acid. This pH adjustment step, that is, the addition of the acid, precipitates protein from the liquid extract phase. It is preferable that the pH of the extract phase be adjusted to 4.5 by the addition of the acid since pH 4.5 is the isoelectric point of the protein, that is the point where the protein is least soluble in water, and at this point therefore the most protein is recoverable from the liquid extract phase by an acid precipitation technique. The precipitated protein is then recovered from the acid-adjusted extract phase by any convenient physical separation method, for example, by centrifugation, producing a liquid supernatant fraction commonly called the whey, and a precipitated protein fraction referred to as the acid curd. The acid curd is then typically neutralized to pH 7 by the addition of a suitable base, such as sodium hydroxide, and then spray dried to produce a dry protein isolate, which usually has a protein concentration of greater than 90%. Typically, the protein isolation procedure is also carried out at temperatures, ranging 20.degree.-45.degree. C.
With the alkaline extraction/acid precipitation procedures of the prior art, however, protein isolates are produced having less than desirable physical characteristics which have limited the suitability of the protein isolate in food applications. The protein isolates produced by the prior art alkaline extraction/acid precipitation procedures, for example, have exhibited objectionable off-colors, odors and flavors, which have severely restricted the applicability of the protein isolate in human food applications. Sunflower protein isolates are particularly notable in this regard, ordinarily having an intense green color which cannot be removed from the isolate product by dialysis or other conventional means of purification. If the intensely colored isolate is added to food products as a protein supplement additive, the green color is imparted to the food product so that it is characterized either by a green cast or by a green color, which is ordinarily considered unappetizing. As a result, protein isolated by conventional techniques from sunflower meal is not ordinarily useful in human food applications. In addition to the problem of green color formation, sunflower protein isolates have also been characterized by an unattractive and grassy flavor.
While the off-color problem is most severe with sunflower protein isolates, less extensive off-colors also occur in all vegetable protein isolates, including soy protein, which have limited to various extents the suitability of such protein isolates in food applications. Many of these protein isolates also suffer from severe off-flavor problems, which have further restricted their use in many food applications. The soy protein industry, for example, has major problems in controlling off-flavors from lipoxygenase-catalyzed lipid oxidation which frequently precludes soy protein utilization in many food applications.
Heretofore, a number of attempts have been made in order to overcome the aforementioned disadvantages of the conventional alkaline extraction/acid precipitation protein isolation technique. U.S. Pat. No. 3,622,556, for example, teaches a modified alkaline extraction/acid precipitation technique wherein green color formation in sunflower protein isolates is minimized by extracting the sunflower meal under an inert gas blanket, such as nitrogen, and then passing the resulting liquid protein extract phase through an ultrafiltration membrane prior to the acid precipitation step. While use of this modified process has resulted in sunflower protein isolates having improved color, this process has required the use of special expensive equipment which has prevented the use thereof on a commercial basis.
In addition, various reducing agents have also been employed during the alkali extraction step in order to minimize off-color formation. Smith et al, Cereal Chemistry, Vol. 25, pages 399-406 (1948), for example, indicates that green color can be temporarily removed from sunflower protein isolates by the use of reducing agents such as dithionate salts during alkali extraction. While protein recovered after the use of this reducing agent may initially be light colored, the green color reappears if the isolated protein is utilized as a supplement in foods with even a slightly basic pH. Similarly, Gheyasuddin et al, Food Technology, Vol. 24, page 242 (1970), discloses that a colorless sunflower protein isolate may be prepared by treating the soluble sunflower protein with sodium sulfite and then washing the protein acid curd with 50% isopropanol. The protein isolate produced by this procedure, however, develops an objectionable brown color at pH's above 7.5.
Various pretreatment operations have also been proposed in the prior art in order to remove the various color-forming and other impurities from the vegetable protein source prior to alkali extraction. The aforementioned Smith, et al, Cereal Chemistry, article, for example, reports the use of hot 70% ethanol and absolute methanol for extracting chlorogenic, caffeic and quinic acids from sunflower meal. It has been found, however, that complete extraction of the color-forming phenolic acids from sunflower meal requires refluxing or shaking for several hours, which has rendered the use thereof undesirable. Various other pretreatments for the purpose of removing color-forming phenols from sunflower meals are also reported in Sodini et al, Journal Agricultural Food Chemistry, Vol. 25, page 822 (1977); Rhee et al, Final Report to U.S.D.A., ARS, Athens, Georgia, Research Agreement No. 12-14-7001-847 (1979); and Bau et al, 182nd National Meeting of American Chemical Society, Division of Agricultural and Food Chemistry, New York, New York (1981). While each of these various pretreatments has proven successful to a certain extent in removing bound color-forming phenolic compounds from the sunflower meal, none of these methods has proven commercially viable.